The object of the present invention is a process for recovering the methanol contained in a liquid hydrocarbon feed.
Patents FR-B-2,605,241 and FR-B-2,636,857 relate to a natural gas treating process allowing to achieve at least dehydration and recovery of the natural gas liquids contained in the gas by temperature lowering. The gas is protected against hydrates formation by addition of methanol. At the end of the cooling stage, three phases are separated in a decantation drum (an aqueous liquid phase, a hydrocarbon liquid phase referred to as  less than  less than natural gas liquids greater than  greater than  or  less than  less than NGL greater than  greater than  and a gas phase consisting of methane), these three phases containing methanol.
For economic reasons, it is advantageous to recover the methanol contained in the hydrocarbon liquid phase.
U.S. Pat. No. 4,014,667 describes a process for recovering the methanol present in a hydrocarbon liquid phase. This process consists in water washing the hydrocarbon liquid phase and it is carried out in a liquid/liquid contacting device, a mixer/contactor for example. After contacting, the hydrocarbon liquid phase is free from almost all of the methanol and it is subjected to a separation operation for separating the hydrocarbons that constitute it. The separation operation can be carried out in a distillation column arranged downstream from the liquid/liquid contacting device.
This water washing process is advantageous in that it is relatively simple, it does not require high installation and operating costs, and it allows to recover a great fraction of the methanol present in the hydrocarbon liquid phase.
However, the process according to patent U.S. Pat. No. 4,014,667 has certain drawbacks. At the outlet of the mixer/decanter, the hydrocarbon liquid phase is water saturated and it has to be subjected to an additional dehydration stage.
Furthermore, liquid/liquid contacting of the hydrocarbon liquid phase with water, downstream from the distillation column, presents at least two drawbacks. On the one hand, contacting can cause cooling of the hydrocarbon phase before it is fed into the distillation column, which requires a higher energy supply in the distillation column to separate the hydrocarbons. On the other hand, during contacting, the hydrocarbon phase fills with water, which increases the amount of material to be processed in the distillation column.
Besides, this water washing process allows to recover only an aqueous solution with a low methanol concentration. It is then necessary to use a fractionating column to separate the methanol from the wash water.
The invention proposes a new process for recovering the methanol contained in a hydrocarbon liquid phase. This process can be applied for processing the natural gas liquids obtained with the processes described in patents FR-B-2,605,241 and FR-B-2,636,857. This new process allows to overcome the aforementioned drawbacks.
The present invention uses the fact that methanol forms azeotropes with propane and butane. The boiling temperature of methanol-propane and methanol-butane azeotropes is lower than that of the propane and butane pure substances and substantially different from that of the other alkanes. It is thus possible to separate by distillation the methanol azeotropes from the other hydrocarbons and notably from propane and butane. The methanol can thus be isolated in a small stream of matter in relation to the stream to be treated.
The present invention aims to isolate the azeotropes formed in a distillation column by methanol, propane and butane. The azeotropes are then liquefied and mixed with water so as to dissolve the methanol in the water. Then the mixture is fed into a decantation tank to separate the aqueous phase from the liquid hydrocarbon phase. Finally, a methanol-containing aqueous phase is discharged and the methanol-depleted hydrocarbon phase is recycled to the distillation column as reflux.
The process according to the invention can be generally defined as a process for recovering the methanol contained in a hydrocarbon liquid feed, comprising the following stages:
a) distilling the hydrocarbon liquid phase containing the methanol to isolate a gas phase consisting of methanol-hydrocarbons azeotropes,
b) mixing said gas phase obtained in stage a) with water,
c) separating the mixture obtained in stage b) into a methanol-rich aqueous phase and a methanol-poor hydrocarbon phase, said hydrocarbon phase being recycled to stage a).
According to the process of the invention, the azeotropes isolated can be methanol-propane and methanol-butane. Said gas phase obtained in stage a) can be either liquefied before stage b), or said gas phase can be liquefied upon mixing with the water during stage b). In stage c), the mixture obtained in stage b) can be separated by decantation. Before stage a), the methane and the ethane can be separated from the hydrocarbon liquid phase by distillation. In stage a), a liquid phase containing the hydrocarbons consisting of more than five carbon atoms and a gas phase containing propane and butane can be obtained.
The process according to the invention can be used to process the methanol-containing hydrocarbon liquid phase obtained after dehydration or dehydration and deacidizing of a raw natural gas.
The process according to the invention, wherein the methanol-rich aqueous phase obtained in stage c) is recycled, can be used in a raw natural gas dehydration or dehydration and deacidizing process.
The process according to the invention affords many advantages in relation to the prior art.
The process according to the invention allows to make significant energy savings. Since no water washing is performed upstream from the hydrocarbon separation column, the hydrocarbon liquid phase is neither cooled nor water saturated. Thus, the distillation column processes a low-flow rate and warmer stream, and it therefore requires less heat for separation of the hydrocarbons and of the azeotropes. Furthermore, according to the process of the invention, mixing of the azeotropes obtained at the top of the distillation column with water allows to reduce the temperature of said azeotropes or even to liquefy them.
Besides, the process according to the invention requires a lower stream of water than the washing process according to the prior art. According to the invention, this water stream is under no obligation to dissolve all of the methanol contained in the azeotropic mixture. In fact, the hydrocarbon liquid phase obtained after mixing with the water is recycled in the process as reflux to the distillation column. A concentration of 10% by mole of methanol in the aqueous phase discharged can thus be obtained.